SOURCE

// 扁平数数组转为json树结构
let flatArr = [{
    id: 1,
    title: "解忧杂货铺1",
    parent_id: 0
},
{
    id: 2,
    title: '解忧杂货铺2',
    parent_id: 0
},
{
    id: 3,
    title: '解忧杂货铺2-1',
    parent_id: 2
},
{
    id: 4,
    title: '解忧杂货铺3-1',
    parent_id: 3
},
{
    id: 5,
    title: '解忧杂货铺4-1',
    parent_id: 4
},
{
    id: 6,
    title: '解忧杂货铺2-2',
    parent_id: 2
},
]

function covert(flatArr) {
    var mapTmp = {}
    for (var i = 0; i < flatArr.length; i++) {
        mapTmp[flatArr[i].id] = flatArr[i]
    }
    // {1:{id:'',title:'',parent_id:0}}

    var reult = []
    flatArr.forEach((ele) => {
        var parent = mapTmp[ele.parent_id]    // 对象{ }，parent就是一个地址
        if (parent) {
            (parent.children || (parent.children = [])).push(ele)
        } else {
            reult.push(ele)
        }

    })
    return reult

}

console.log('flatArr',covert(flatArr) )